Two-stroke internal combustion engine

ABSTRACT

A two-stroke internal combustion engine in which the gas exchange is controlled by means of inlet and exhaust ports or valves in conjunction with one or two flat pistons, whereby the flank portions adjoining the upper edge of the parts are so constructed that the inlet ports form together with the peripheral surface of the piston or with an insert, a nozzle that directs from the beginning of the opening the inflowing fresh gas jets over the piston head in a substantially flat manner.

United States Patent Papst 3,695,239 51 Oct. 3, 1972 TWO-STROKE INTERNAL COMBUSTION ENGINE [72] Inventor: Hermann E. R. Papst, 7742 St.

Georgen, Schwarzw, Germany [22] Filed: Sept. 10, 1970 21 Appl. No.: 70,992

[] Foreign Application Priority Data Sept. 10, 1969 Germany ..P 19 720.6

[52] US. Cl ..125/ PD, 123/65 P, 123/65 W [51] Int. Cl ..F02b 25/14 [58] Field of Search ..123/65 P, 65 PD, 65 W [56] References Cited UNITED STATES PATENTS 2,573,989 l/ll Sammons et al 123/65 PD 2,198,679 4/1940 Radelet et al ..123/65 PD 2,390,637 11/1942 Brill ..l23/65W 2,195,044 3/1940 Zahren ..123/65 W 1,728,472 9/1929 Brekke ..123/65 W 2,638,081 5/1953 Spannhake ..123/65 P FOREIGN PATENTS OR APPLICATIONS 790,355 11/1935 France ..123/65 W 466,547 Ill 1951 Italy ..123/65 P Primary Examiner--Wendell E. Burns Attorney-Craig and Antonelli [57] ABSTRACT A two-stroke internal combustion engine in which the gas exchange is-controlled by means of inlet and ex haust ports or valves in conjunction with one or two flat pistons, whereby the flank portions adjoining the upper edge of the parts are so constructed that the inlet ports form together with the peripheral surface of the piston or with an insert, a nozzle that directs from the beginning of the opening the inflowing fresh gas jets over the piston head in a substantially flat manner.

42 Claims, 10 Drawing Figures PATENTEDHBI-s m2 VIP ' I INVENTOR HERMANN PAPST Aflto ezu, stwwumu ATTORNEYS TWO-STROKE INTERNAL COMBUSTION ENGINE The invention relates to a two-stroke internal combustion engine, preferably having an inserted cylinder liner, in which the gas exchange is controlled by inlet ports and exhaust ports or valves in conjunction with one or two flat pistons.

In the known two-stroke combustion engines in which the gas exchange operation is effected with the aid of port or slot control, the fresh gas enters the cylinder in the form of thin scavenging jets at an angle of about 45 in relation to the longitudinal axis of the piston when the edge of the piston head frees the upper cross-section of the inlet port on moving over the upper edge of this port. This direction of gas flow is determined by the edges and bounding surfaces of the inlet cross section which vary with the movement of the piston, namely by the piston skirt surface and the upper terminal surface of the port. In consequence of this arrangement, at the commencement of the opening of the inlet ports the scavenging jets cut a conical core out of the volume of used gas contained in the cylinder, disposed above the piston head, which is very detrimental to the scavenging of the cylinder. During the further movement of the piston which opens the inlet port, the cone of fresh gas formed above the piston is gradually destroyed, while the fresh gas jets entering through the inlet ports during the opening operation mix with the cone of used gas which has remained above the piston. In the prior art arrangements the new cylinder charge therefore always contains a relatively high proportion of used gas. In double-piston'engines equipped with opposed flat pistons and working on the two-stroke principle, despite end-to-end scavenging with a large excess of air, the best efficiency consequently amounted to only about 86%.

Even in the single-piston two-stroke engines, for example with reverse scavenging, in consequence of the initial inclined jet formation, the scavenging action is poor and in addition the loss of fresh gas is considerable. Moreover, after the compression some of the fresh gas may escape unused from'the charge ignited by the spark plug, as the result of turbulence or separation.

Another disadvantage of the known scavenging methods in two-stroke engines is caused by the fact that the used gas lying behind the lands between theinlet ports is not seized and pushed away by the incoming current of fresh gas. These blind zones of the used gas are therefore swirled up with the incoming fresh gas only in the later course of the change of charge. It may be assumed that these parts of the used gas likewise remain in the fresh charge. A further disadvantage of the heretofore known arrangement and constructions of the inlet slots or ports, especially with rounded-off and circular configuration, results from the fact that the optimum slot or port width is not available at the commencement of the opening of the slot or port. The entering fresh gas jet is initially strongly throttled. Only after a few millimeters of piston travel the slots or ports become effective with their entire width. As a result of these circumstances, two-stroke engines have heretofore had a power far lower than the theoretically attainable double power compared with four-stroke engines of the same size.

The problem underlying the invention consists in eliminating the disadvantages of known two-stroke internal combustion engines and in improving the exchange of the charge in two-stroke engines from the beginning to the end of the change, a particular object of the invention being the provision of a more favorable 5 jet arrangement in order to remove by means of a layer of fresh gas, from the commencement of the admission or inflow, the residue of used gas situated above the piston head and not removed in the existing prior art scavenging arrangements, thereby considerably improving the scavenging efficiency.

According to the invention, this problem is solved by the fact that through the construction of the flank portion adjoining the upper edge of the port, the inlet ports or slots form together with the peripheral surface of the flat piston or with an annular inserted part, at the commencement of the opening, a nozzle which from the outset guides the incoming jets of fresh gas onto the piston head. The imaginary continuation of the center line (symmetrical, bisector) between the flank portions forming the inlet duct or channel consequently runs parallel to or at a small, acute angle to the piston surface even on the first opening of the inlet ports.

in addition, at least the flank portion adjoining the opening, upper control edge of the inlet ports can advantageously form an acute angle of less than 45 with the adjoining working surface of the cylinder and can be in the form of a hollow trough. A

The inlet ports and preferably also the exhaust openings may be so shaped that at the commencement of the change of charge, the maximum' width of the opening cross-section is effective at the inlet or outlet, while the ports may be disposed in a ring around the cylinder liner in such a manner that the total width of their intermediate lands at the commencement of the opening amounts to only a minor fraction of the periphery of the cylinder. It is very advantageous in thatconnection if the working surface of the piston forms an obtuse angle with flank portions of the cylinder ports so that the inflowing fresh gas jet is deflected flat over the piston top. Preferably the aforementioned flank portions are constructed as hollow trough. i

The inlet ports may advantageously be disposed in a ring around the cylinder liner and may have an angular, non-rounded profile. Through the use of a large number of ports of angular, non-rounded profile, e.g.,

of rectangular profile, the jet is from the outset kept as wide as is permissible. However, it is also possible to provide more ports with reduced width of the lands. In this construction of the lands in the form of narrow profiles or guide blades between the ports, the formation of blind zones occupied by used gas between the ports is considerably reduced.

The nozzle formed in this arrangement by the outer piston surface uncovered in the port and by the upper flank of the latter has the effect that the incoming current or jet of fresh gas extends approximately parallel to the piston head surface or encounters the latter at only a small acute and harmless angle, so that from the outset the cone of used gas is only very small or no longer occurs at all by reason of the Coanda-or Rottaeffects. The incoming jet is at the same time largely protected against swirling up with the used gas by lying against the pistontop.

According to another very advantageous embodiment of the invention, in order to achieve a quicker gas exchange, the exhaust ports may have a triangular or trapezoidal cross-sectional shape which is first opened by the piston at the greatest width. In this case it is very convenient for the flanks of the outlet ports which open first, to form an acute angle with the working surface of the cylinder. The lands which are acted on by the hot gas, and which remain in the cylinder as triangular or trapezoidal guide prongs, permit in their increasing cross-section a very good transfer of heat to the adjoining cylinder wall. Above the piston head, for example in the case of double piston engines with end-to-end scavanging, the fresh gas forms a disc which becomes thicker in the center and displaces the used gas in the upward direction. In addition, because of the narrow lands or profiles and the widening jets of fresh gas, only very restricted blind zones for exhaust gases are formed, so that the scavenging effect is considerably improved. lf the ports are disposed so as to extend slightly obliquely in relation to the cylinder wall, it is also possible to obtain a swirling effect which is advantageous for the scavenging of the cylinder.

It is advantageous for the inlet ports or outlet ports to be manufactured with a disc-shaped tool, for example by means of a disc milling cutter. A corresponding hollow profile of a high speed disc milling cutter and its uniform cutting through the inner surface of the cylinder enable a straight edge to be obtained and provide thescavenging jets with an action which is uniform at all points.

In order to intensify the action of applying the air or air-fuel mixture jets, which enter the interior of the cylinder, against the piston surface, it is advantageous for the edge of the piston head to be slightly bevelled or curved outwardly in order to enable the inlet nozzle jet to be applied against it.

According to another very advantageous feature of the invention, a thin ring provided with tongues, which are bent over approximately perpendicularly into the inlet ports and which form slit nozzles with the inner upper edge flank of the inlet ports, can be disposed around the cylinder in the upper region of the inlet ports which opens first.

A still greater improvement of scavenging can be achieved according to the invention by providing the cylinder, at the height of the inner top edge of the ports, with an internal anular groove or an incision, or by dividing the cylinder by means of an annular gap which is perpendicular to the axis of the cylinder and which is smaller in the axial direction of the cylinder than the height of a piston ring of the piston running in the cylinder. The portion of the cylinder which is provided with the ports is thus spaced apart from the actual working cylinder part by a short distance which forms a narrow circular slot in the cylinder surface, so that the fresh gas can immediately enter the cylinder over the edge of the piston and over the greatest possible width around the latter. In this arrangement it is necessary that the distance between the cylinder walls be shorter than the width of the piston rings.

In this arrangement it is very convenient for the annular gap on the bottom cylinder part to be bounded by a ring which rests on the lands between the inlet ports or is inserted into the incision groove. This ring, which for example is made of sheet metal, forms together with the plane or curved end face of the working cylinder part, in which takes place the compression and combustion, a very'favorable nozzle profile for the incoming fresh gas jets. The thin ring may, for example, be fastened on the port lands by adhesive bonding or spot welding, or may be held in the groove by spring action. The annular slit nozzle formed in this manner introduces the fresh gas from the outset between the piston head and the used gas and thus detaches the used gas from the piston while avoiding turbulence. After the entry of the separating layer, as the piston moves further back, the elongated, preferably inclined ports distributed around it are opened. This brings about an intensive swirling in the fresh gas. As the piston moves back after the change of charge, the narrow annular slot remains open to the full extent to the last moment. The scavenging gas then flows in more slowly through the narrow annular slots and is applied against the cylinder wall approximately in laminar form, By means of the annular slit nozzle the greatest possible time is thus achieved for the admission of fresh gas. This is helpful in improving the efficiency of the two-stroke engine, because the relative port height can be made smaller and the expansion can be lengthened.

' It may also be appropriate to provide the ring with guide and spacing ribs. These narrow swirling guide ribs or guide lands in the nozzle cross-section may, for example, be pressed out of the material of the ring. In order to ensure still better scavenging, it is advantageous for a plurality of rings with guide ribs to be disposed one above the other between the cylinder parts. In this arrangement it is necessary that the cylinder parts should be held exactly in alignment with one another through the external centering in the surrounding cylinder block.

It is advantageous for the working cylinder to be provided at the end faces for admission and for exhaust with an acute-angled edge, which may pass over into a concave surface.

It is also very advantageous for the cylinder to be divided at the height of the controlling top edge of the exhaust ports by gaps which in the axial direction are smaller than the height of a piston ring. This division of the cylinder parts prevents thermal expansions from deforming the cylinder wall. The expansion of the lands through heating is possible without transverse stressing. The piston rings can without difficulty run over the gaps at their tip, because the cylinder parts are centered in relation to one another in the cylinder block.

It is appropriate for the last of the parts of a divided cylinder, which are provided with profiled inlet or exhaust ports, to be made of a lighter metal and/or a metal which ensures better removal of the heat than the expansion part of the cylinder. The sliding surfaces thus remain wetted with oil.

It is very appropriate for the edges bounding the inlet and/or exhaust ports and directed oppositely to the flow to be rounded in order to increase the flow coefficient. Particularly in the case of an axially divided cylinder, the cylindrical part which is provided with the inlet and/or exhaust ports, and which is correspondingly profiled on its end face, may be manufactured by a shell molding process.

ring surrounding'the latter according to the invention,

FIG. 4A is a side view showing a piston for use with the invention which has bevelled upper edges,

FIG. 5 is a cross-section of the cylinder illustrated in FIG.'4,

FIG. 6 shows part of a divided cylinder with an attached flat ring,

FIG. 7 shows part of a divided cylinder with a ring provided with guide ribs,

FIG. 8 shows part of a divided cylinder with two inserted rings, and

FIG. 9 is longitudinal section through separated cylinder parts in the region of the exhaust ports.

In FIG. 1 there is shown a cylinder 1 of known construction, with end-to-end scavenging. The ports 2 disposed side by side on the periphery are rounded 'at their top end 3. The piston 4, which moves in the direction of the arrow, frees the ports 2 at their rounded end 3 by its top edge 5,-and the fresh gas 6 flows obliquely under pressure into the cylinder 1. The flow of the fresh gas 6 in the cylinder 1 is indicated by broken lines, and it is determined mainly by the bounding walls which form in width a nozzle, namely by the free parts of the piston wall 7 and of the top flank3 of the ports 2. The direction of flow of the fresh gas 6 is therefore approximately at 45 to the piston head surface 5.

The exhaust gas cone 8 forming above the piston head is at first not acted on by the fresh gas 6. In the cylinder 1 newly filled with fresh gas there is consequently contained a large exhaust gas component which, as has already been stated above, greatly reduces scavenging efficiency. As the piston 4 moves further downwards and thus the piston edge 5 further frees the ports 2, the fresh gas enters the cylinder 1 at an increasingly flatter angle to the piston head, so that the used gas present there is also caught up by the fresh gas but is now no longer removed from the cylinder.

Another disadvantage of the known two-stroke scavenging can be seen from FIG. 2. This Figure shows a cross-section through the cylinder 1 in FIG. 1. The fresh gas 6 entering the cylinder 1 through the ports 2 cannot immediately act on the pocket of used gas 10, disposed behind the lands 9 between the ports 2. Only in the subsequent course of the exchange of the charge are these blind zones of used gas swirled up with the fresh charge. Parts of this 'used gas also remain in the fresh charge so that scavenging efficiency is also reduced thereby.

In FIG. 1 it can also be seen how the top piston edge 5 frees the rounded ends 3 of the ports 2, so that at the commencement of the opening the width of the ports is smaller than at a later instant of time. The inlet jet of fresh gas 6 is therefore substantially throttled as it enters at the beginning. Only after a certain movement of the piston are the ports 2 effective over their entire width.

FIG. 3 shows a longitudinal section through a cylinder 11 according to the invention. As can be seen in FIG. 3, the inlet ports 12 open immediately over their full width. The product of cross-section and opening time is therefore substantially more favorable. This applies both to the beginning and to the end of the gas exchange, and it is precisely the upper, wider parts of the port cross sections which are opened longest. The inlet ports 12 are disposed substantially parallel to the cylinder axis in the wall of the cylinder 11 and, since they are manufactured by means of a disc milling cutter having a slightly hollow profile, they form at the upper port flank 13 a very acute angle with the inner working surface 14 of the cylinder. The nozzle formed by the outer piston surface 15 uncovered in the port and the upper port flank 13 therefore imparts to the incoming jet 16 only a slight, harmless inclination in relation to the piston top 17. From the commencement the used gas cone is thus very small or is no longer formed at all. The incoming jet 16 is effectively guided against and along the piston top 17 and thus to a large extent is protected from swirling up with the used gas.

From FIG. 3 it can be seen that the angle enclosed by the port flank 13 and the cylinder wall 14 is very acute, the flank 13 preferably being in the form of a hollow trough. The nozzle opened as the piston moves over the top edge of the port 12 therefore has a jet direction which extends very close to the piston head 17. In accordance with known flow phenomena the air or gas mixture jets l6 widening out radially inwards will bear against the piston head surface. This can be assisted if the edge of the piston head is provided with flat bevelling or is slightly curved in the outward direction. Above the piston head 17 the fresh gas forms a disc which becomes thicker in the middle, and displaces the used gas in the upward direction.

FIGS. 4 and 5 show a two-stroke engine cylinder with reverse scavenging, and with inlet ports which are nonradially aligned and slightly curved in their longitudinal direction. These port troughs can, for example, be produced by narrow disc milling cutters or by eroding. In FIG. 4 it can also be seen that the cylinder is provided with a ring 18 or with a sheet metal strip with side tongues 19, the tongues 19 being bent radially to engage into the individual ports 20. The first incoming scavenging jets 21 are thereby additionally guided onto the piston head, and through the action of the nozzles 23 formed by the port flank 22 and the ring l8, 19, the incoming fresh gas jets 21 flow flat over or at a flat angle to the piston head 24 of the piston 25 in such a manner that a used gas cone can no longer be formed.

FIG. 4A is a side view of a piston to be used with the present invention which has upper bevelled edges.

According to the invention a further improvement can be achieved in two-stroke engines by subdividing the cylinder along its axis. Such a cylinder is illustrated in FIG. 6. The portion 27 provided on the outside with the inlet ports 26, which are in the form of hollow grooves, is thus slightly spaced apart axially from the actual working cylinder part 28, in order that the fresh gas can immediately enter the cylinder over the greatest possible width around the latter over the piston edge, which is not shown in FIG. 6. In this arrangement it is necessary that the distance 29 between the two cylinder parts 27, 28 should be shorter than the width of the piston rings. The end face of the working cylinder 28, in which the compression and combustion take place, is plane or bevelled in such a manner that together with the annular flat part 30 disposed on the bottom cylinder part 27, a favorable nozzle profile is formed. The thin ring 30 lies on the port lands 31 and is fastened for example by adhesive bonding or spot welding. The annular slit nozzle 32 formed in this manner introduces the fresh gas from the outset between the piston head and the used gas and thus lifts the used gas off the piston while avoiding swirling phenomena. As the piston moves further back, the inclined ports 26 disposed around the periphery are opened. Intensive swirling is thus produced in the fresh gas. As the piston moves back after the exchange of the charge, the narrow ring slot 32 remains open over the entire periphery of the cylinder to the last moment. The largest possible time period for the admission of fresh gas is thus obtained through the annular slit nozzle 32.

If necessary, the ring 30 may also be provided with additional swirling guide lands 33 in the nozzle cross section 32, these lands determining the spacing width of the nozzle and being, for example, pressed out of the material of the ring. This is illustrated in FIG. 7.

FIG. 8 shows the arrangement of two rings 30, one above the other. The nozzle cross-section 32 is subdivided axially by the guide lands 33, which atthe same time serve for the connection, by welding, to the port lands 31.

The cylinder parts 27 and 28 are held in exact alignment with one another by external centering means 34 in the surrounding cylinder block (FIG. 6). As a rule, the fresh air or fresh gas entering around the periphery should form only a separating air disc without rotation between the piston head and the spent gas. Swirling between fresh gas and spent gas is thus reduced. As the piston moves further downwards, the port ducts 26 in the lower cylinder part 27 increase the admission crosssection. The fresh gas however continues to flow more slowly through the annular slit nozzle 32 until the inlet opening is closed again. The ports 26 in the bottom cylinder parts 27 are inclined in relation to the cylinder axis as required, in order to produce swirling in the fresh gas. The individual jet directions may differ. It is thus possible with greater certainty to ensure that the entire cross-section of the cylinder will be uniformly filled with fresh gas. This fresh gas then rotates as a swirling ring in front of the injection nozzles, whereby mixture formation and combustion are improved in known manner.

FIG. 9 shows a longitudinal section through a cylinder 28 and the cylinder 35 situated therebeneath and provided with trapezoidal outlet ports for the discharge of the spent gas. The transversely divided cylinder and the shape of the ports 36 once again provide the greatest possible width immediately on the opening of the ports 36. The lands 37 situated in the cylinder part 35 may be triangular or trapezoidal, in order that they may transfer the heat in the increasing cross-section with a slight gradient to the adjoining cylinder wall. The separation of the cylinder parts 28, 35, which have a gap between them, prevents the ther- 28, 35 are held centered in relation to one another in a cylinder block.

The trapezoidal openings 36 in the wall of the cylinder part 35 may have flanks which initially yield against the gas current, in order that the exhaust gas may heat the outside of the cylinder as little as possible as long as it passes out of the nozzle gap at high speed, up to the speed of sound.

In the case of a divided engine cylinder the exhaust part 35 is conveniently made of a material which has a better thermal conductivity than cast iron, in orderthat excessive heating of the triangular and trapezoidal lands 37 guiding the piston rings and the piston may be avoided. A still better removal of heat can be achieved in this manner than is obtained by the widened shape of the lands. It is advantageous for the cylinder part equipped with the ports for the exchange of charge also to be given an increased clearance in relation to the peripheral wall of the piston. This is immediately possible because the pistons do not require any special sealing during the change of charge. The bottom cylinder part 35 may, for example, be made of a light metal or copper alloy. The removal of heat from the lands 37 made of this material then is realized improved by a multiple compared, for example, to the case of lands of cast iron. The temperature of the lands 37 necessary for guiding the piston rings is therefore substantially reduced, so that they remain covered with lubricant and the limit power of the engine is raised.

Through the improvement of the gas exchange according to the invention and through increased cylinder filling it is therefore possible to obtain higher speeds, because the opening time-cross-sections for the exchange of the charge are increased in an optimum manner at the inlet and exhaust ports, particularly at the beginning and at the end. The power of a twostroke engine according to the invention can in a simple manner be almost doubled for given engine speeds in comparison with a four-stroke engine, particularly in the case of air scavenging.

While I have shown and described several embodiments in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to those skilled in the art, and I therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

What I claim is:

l. A two-stroke internal combustion engine of the type wherein the gas exchange is controlled by means of inlet port means and exhaust port means in conjunctionwith at least one piston; said engine comprising: cylinder means having an upper and lower end, a piston arranged for reciprocating up and down movement within said cylinder means, said piston having a piston head portion with a substantially flat upper surface at the upper end thereof, inlet port means including at least one inlet port opening arranged in said cylinder means for cooperating with said piston to introduce fresh gas to said cylinder means upon movement of said piston head portion below the upper edge of said at least one opening, wherein the upper edge of said at least one opening and the upper peripheral edges of the head portion are so constructed that fresh gas initially entering the upper part of said at least one opening is directed substantially toward the upper surface of said head portion, and gas guide means arranged immediately below the upper edge of said at least one opening for radially guiding the initially entering fresh gas, said gas guide means including at least one tongue projecting into said at least one opening approximately perpendicularly to the longitudinal axis of the cylinder means.

2. An internal combustion engine according to claim 1, wherein an annular ring means surrounds said cylinder means in a position closely spaced below said at least one opening, and wherein said at least one tongue is mounted at the upper side of said ring means such that said at least one tongue extends from said ring means upwardly to the upper edge of said at least one opening, whereby said ring means and said at least one tongue together with the upper edge of said at least one opening delimit nozzle openings for the initially entering fresh gas.

3. An internal combustion engine according to claim 2, wherein the lower edge of said at least one opening is spaced from the lowermost extent of said ring means by an amount substantially greater than the distance between said ring means and the upper edge of the at least one opening, and wherein said at least one opening is uninterrupted from the bottom of said ring means to the lower edge of said at least one opening, whereby fresh gas introduced after initial opening of said at least one opening is introduced into said cylinder means with a minimum of turbulence to reduce mixing of said fresh gas with exhaust gas remaining in the cylinder means.

4. An internal combustion engine according to claim 1, wherein a plurality of inlet port openings are provided, each of said inlet port openings having at least one tongue projecting therein.-

5. An internal combustion engine according to claim 2, wherein a plurality of inlet port openings are provided, each of said inlet port openings having at least one tongue projecting therein.

6. An internal combustion engine according to claim 5, wherein said inlet port openings are asymmetrically arranged on the circumference of said cylinder means, and wherein exhaust port means are provided along the same vertical plane as said inlet openings which include at least one exhaust port opening arranged asymmetrically with respect to said inlet openings, whereby reverse scavenging takes place upon operation of the engine.

7. An internal combustion engine according to claim 3, wherein a plurality of inlet port openings are provided, each of said inlet port openings having at least one tongue projecting therein.

8. An internal combustion engine according to claim 7, wherein said inlet port openings are asymmetrically arranged on the circumference of said cylinder means, and wherein exhaust port means are provided along the same vertical plane as said inlet openings which include at least one exhaust port opening arranged asymmetrically with respect to said inlet openings, whereby reverse scavenging takes place upon operation of the engine.

9. A piston-cylinder arrangement for use on a twostroke internal combustion engine of the type wherein the gas exchange is controlled by means of inlet port means and exhaust port means in conjunction with at least one piston, said arrangement comprising: cylinder means having an upper and lower end, a piston arranged for reciprocating up and down movement within said cylinder means, said piston having a piston head portion with a substantially flat upper surface at the upper end thereof, inlet port means including at least one inlet port opening arranged in said cylinder means for cooperating with said piston to introduce fresh gas to said cylinder means upon movement of said piston head portion below the upper edge of said at least one opening, wherein the upper edge of said at least one opening and the upper peripheral edges of the head portion are so constructed that fresh gas initially entering the upper part of said at least one opening is directed substantially toward the upper surface of said head portion, and gas guide means arranged immediately below the upper edge of said at least one opening for radially guiding the initially entering fresh gas, said gas guide means including at, least one tongue projecting into said at least one opening approximately perpendicularly to the longitudinal axis of the cylinder means.

l0.- An arrangement according to claim 9, wherein an annular ring means surrounds said cylinder means in a position closely spaced below said at least one opening, and wherein said at least one tongue is mounted at the upper side of said ring means such that said at least one tongue extends from said ring means upwardly to the upper edge of said at least one opening, whereby said ring means and said at least one tongue together with the upper-edge of said at least one opening delimit nozzle openings for the initially entering fresh gas.

11. An arrangement according to claim 10, wherein a plurality of inlet port openings are provided, each of said inlet port openings having at least one tongue projecting therein.

12. An arrangement according to claim 11, wherein said inlet port openings are asymmetrically arranged on the circumference of said cylinder means, and wherein exhaust port means are provided along the same vertical plane as said inlet openings which include at least one exhaust port opening arranged asymmetrically with respect to said inlet openings, whereby reverse scavenging takes place upon operation of the engine.

13. A two-stroke internal combustion engine according to claim 4, characterized in that the cylinder means includes a cylinder liner.

14. A two-stroke internal combustion engine according to claim 1, characterized in that the engine includes two substantially flat pistons.

15. A two-stroke internal combustion engine according to claim 1, characterized in that at least that part of the flank portion adjoining an opening upper control edge of the at least one opening forms an acute angle with an adjoining working surface of the cylinder.

16. A two-stroke internal combustion engine according to claim 15, characterized in that the last-mentioned flank portion is in the form of a hollow trough.

17. A two-stroke internal combustion engine according to claim 2, characterized in that the inlet port openings are so shaped that they open and close with their maximum width in the direction of the periphery of the cylinder.

18. A two-stroke internal combustion engine accord ing to claim 17, characterized in that the exhaust port means are also so shaped that they open and close with their maximum width in the direction of the periphery of the cylinder.

19. An internal combustion engine according to claim 17, characterized in that the inlet port openings exhibit a partial disc-shaped slightly concave profile cross-section when viewed from a direction perpendicular to both the cylinder axis and respective radially extending centerlines, and in that said openings are larger on the outside of the cylinder means than on the inside.

20. A two-stroke internal combustion engine according to claim 17, characterized in that the inlet port openings are disposed in a ring around a cylinder liner in such a manner that the total width of their intermediate lands at the commencement of the opening amount to only a small fraction of the periphery of the cylinder means.

21. An internal combustion engine according to claim 17, characterized in that in order to enable the initial fresh gas to impinge more effectively against the upper surface of the piston head portion, the piston head portion is bevelled off at its peripheral edges.

22. A two-cycle internal combustion engine according to claim 17 characterized in that in order to enable the initial fresh gas to impinge more effectively against the upper surface of the piston head portion, the piston head portion is slightly curved outwardly.

23. An internal combustion engine according to claim 20, characterized in that at the height of the inner top edge of the inlet port openings, the cylinder means is divided into two parts by an annular gap extending substantially perpendicular to the axis of the cylinder means.

24. An internal combustion engine according to claim 23, characterized in that the annular gap is delimited on the lower cylinder part by at least one flat annular member which rests on lands between the inlet port openings.

25. A two-cycle internal combustion engine according to claim 14, characterized in that guide and spacer rib means forming part of said gas guide means are arranged at the annular member.

26. A two-cycle internal combustion engine according to claim 25, characterized in that several such annular members provided with guide rib means are arranged one above the other between the cylinder parts.

27. An internal combustion engine according to claim 23, characterized in that one of the cylinder parts is a working cylinder part and in that the working cylinder part is provided at its end faces with a profile ending in an acute angle for the admission and exhaust.

28. A twocycle internal combustion engine according to claim 6, characterized in that the at least one exhaust port opening has an at least approximately triangular cross-section which is opened first by the piston at the larger width thereof.

I 29. A two-cycle internal combustion engine according to claim 6, characterized in that the at least one exhaust port opening has an at least approximately trapezoidally shaped cross-section which is opened first by the piston at the larger width thereof. I

30. An internal combustion engine according to claim 1, characterized in that at the height of a controlling upper edge of the exhaust por means, the

cylinder is divided axially by gaps perpendicular to the axis of the cylinder, the gap in the axial direction being smaller than the height of a piston ring.

31. An internal combustion engine according to claim 30, characterized in that the last of the parts of a divided cylinder, which are provided with profiled inlet or exhaust port means, is made of a metal which has better thermal conductivity than the expansion part of a working cylinder means.

32. A two-cycle internal combustion engine according to claim 31, characterized in that said last part is made of a metal which is lighter in weight and has a better thermal conductivity than cast-iron.

33. 'A two-cycle internal combustion engine according to claim 1, characterized in that edges delimiting the inlet and exhaust port means are rounded off.

34. An internal combustion engine according to claim 1, characterized in that particularly in the case of an axially divided cylinder, the part of a cylinderwhich is provided with the port means, and which is correspondingly profiled on its end face is manufactured by a shell molding process.

35. A two-stroke internal combustion engine according to claim 17, characterized in that the exhaust port means are also so shaped that they open and close with their maximum width in the direction of the periphery of the cylinder means.

36. A two-stroke internal combustion engine according to claim 13, characterized in that the inlet port openings are disposed in a ring around the cylinder liner in such a manner that the total width of their intermediate lands at the commencement of the opening amounts to only a small fraction of the periphery of th cylinder means.

37. An internal combustion engine according to claim 1, characterized in that in order to enable the initial fresh gas to impinge more effectively against the upper surface of the piston head portion, the piston head portion is bevelled off at its peripheral edges.

38. An internal combustion engine according to claim 1, characterized in that at the height of the inner top edge of the inlet port means, the cylinder is divided into two parts by an annular gap substantially perpendicular to the axis of the cylinder, one of said cylinder parts being a working cylinder part.

39. An internal combustion engine according to claim 38, characterized in that the annular gap is delimited on the lower cylinder part by at least one flat annular member which rests on the lands between the inlet port openings.

40. A two-cycle internal combustion engine according to claim 39, characterized in that guide and spacer rib means forming part of the gas guide means are arranged at the annular member.

41. A two-cycle internal combustion engine according to claim 39, characterized in that several such annular members provided with guide rib means are arranged one above the other between the cylinder parts.

42. An internal combustion engine according to claim 38, characterized in that the working cylinder part is provided at the end faces for the admission and exhaust, with a profile ending in an acute angle. 

1. A two-stroke internal combustion engine of the type wherein the gas exchange is controlled by means of inlet port means and exhaust port means in conjunction with at least one piston; said engine comprising: cylinder means having an upper and lower end, a piston arranged for reciprocating up and down movement within said cylinder means, said piston having a piston head portion with a substantially flat upper surface at the upper end Thereof, inlet port means including at least one inlet port opening arranged in said cylinder means for cooperating with said piston to introduce fresh gas to said cylinder means upon movement of said piston head portion below the upper edge of said at least one opening, wherein the upper edge of said at least one opening and the upper peripheral edges of the head portion are so constructed that fresh gas initially entering the upper part of said at least one opening is directed substantially toward the upper surface of said head portion, and gas guide means arranged immediately below the upper edge of said at least one opening for radially guiding the initially entering fresh gas, said gas guide means including at least one tongue projecting into said at least one opening approximately perpendicularly to the longitudinal axis of the cylinder means.
 2. An internal combustion engine according to claim 1, wherein an annular ring means surrounds said cylinder means in a position closely spaced below said at least one opening, and wherein said at least one tongue is mounted at the upper side of said ring means such that said at least one tongue extends from said ring means upwardly to the upper edge of said at least one opening, whereby said ring means and said at least one tongue together with the upper edge of said at least one opening delimit nozzle openings for the initially entering fresh gas.
 3. An internal combustion engine according to claim 2, wherein the lower edge of said at least one opening is spaced from the lowermost extent of said ring means by an amount substantially greater than the distance between said ring means and the upper edge of the at least one opening, and wherein said at least one opening is uninterrupted from the bottom of said ring means to the lower edge of said at least one opening, whereby fresh gas introduced after initial opening of said at least one opening is introduced into said cylinder means with a minimum of turbulence to reduce mixing of said fresh gas with exhaust gas remaining in the cylinder means.
 4. An internal combustion engine according to claim 1, wherein a plurality of inlet port openings are provided, each of said inlet port openings having at least one tongue projecting therein.
 5. An internal combustion engine according to claim 2, wherein a plurality of inlet port openings are provided, each of said inlet port openings having at least one tongue projecting therein.
 6. An internal combustion engine according to claim 5, wherein said inlet port openings are asymmetrically arranged on the circumference of said cylinder means, and wherein exhaust port means are provided along the same vertical plane as said inlet openings which include at least one exhaust port opening arranged asymmetrically with respect to said inlet openings, whereby reverse scavenging takes place upon operation of the engine.
 7. An internal combustion engine according to claim 3, wherein a plurality of inlet port openings are provided, each of said inlet port openings having at least one tongue projecting therein.
 8. An internal combustion engine according to claim 7, wherein said inlet port openings are asymmetrically arranged on the circumference of said cylinder means, and wherein exhaust port means are provided along the same vertical plane as said inlet openings which include at least one exhaust port opening arranged asymmetrically with respect to said inlet openings, whereby reverse scavenging takes place upon operation of the engine.
 9. A piston-cylinder arrangement for use on a two-stroke internal combustion engine of the type wherein the gas exchange is controlled by means of inlet port means and exhaust port means in conjunction with at least one piston, said arrangement comprising: cylinder means having an upper and lower end, a piston arranged for reciprocating up and down movement within said cylinder means, said piston having a piston head portion with a substantially flat upper surface at the upper end thereof, inlet Port means including at least one inlet port opening arranged in said cylinder means for cooperating with said piston to introduce fresh gas to said cylinder means upon movement of said piston head portion below the upper edge of said at least one opening, wherein the upper edge of said at least one opening and the upper peripheral edges of the head portion are so constructed that fresh gas initially entering the upper part of said at least one opening is directed substantially toward the upper surface of said head portion, and gas guide means arranged immediately below the upper edge of said at least one opening for radially guiding the initially entering fresh gas, said gas guide means including at least one tongue projecting into said at least one opening approximately perpendicularly to the longitudinal axis of the cylinder means.
 10. An arrangement according to claim 9, wherein an annular ring means surrounds said cylinder means in a position closely spaced below said at least one opening, and wherein said at least one tongue is mounted at the upper side of said ring means such that said at least one tongue extends from said ring means upwardly to the upper edge of said at least one opening, whereby said ring means and said at least one tongue together with the upper edge of said at least one opening delimit nozzle openings for the initially entering fresh gas.
 11. An arrangement according to claim 10, wherein a plurality of inlet port openings are provided, each of said inlet port openings having at least one tongue projecting therein.
 12. An arrangement according to claim 11, wherein said inlet port openings are asymmetrically arranged on the circumference of said cylinder means, and wherein exhaust port means are provided along the same vertical plane as said inlet openings which include at least one exhaust port opening arranged asymmetrically with respect to said inlet openings, whereby reverse scavenging takes place upon operation of the engine.
 13. A two-stroke internal combustion engine according to claim 4, characterized in that the cylinder means includes a cylinder liner.
 14. A two-stroke internal combustion engine according to claim 1, characterized in that the engine includes two substantially flat pistons.
 15. A two-stroke internal combustion engine according to claim 1, characterized in that at least that part of the flank portion adjoining an opening upper control edge of the at least one opening forms an acute angle with an adjoining working surface of the cylinder.
 16. A two-stroke internal combustion engine according to claim 15, characterized in that the last-mentioned flank portion is in the form of a hollow trough.
 17. A two-stroke internal combustion engine according to claim 2, characterized in that the inlet port openings are so shaped that they open and close with their maximum width in the direction of the periphery of the cylinder.
 18. A two-stroke internal combustion engine according to claim 17, characterized in that the exhaust port means are also so shaped that they open and close with their maximum width in the direction of the periphery of the cylinder.
 19. An internal combustion engine according to claim 17, characterized in that the inlet port openings exhibit a partial disc-shaped slightly concave profile cross-section when viewed from a direction perpendicular to both the cylinder axis and respective radially extending centerlines, and in that said openings are larger on the outside of the cylinder means than on the inside.
 20. A two-stroke internal combustion engine according to claim 17, characterized in that the inlet port openings are disposed in a ring around a cylinder liner in such a manner that the total width of their intermediate lands at the commencement of the opening amount to only a small fraction of the periphery of the cylinder means.
 21. An internal combustion engine according to claim 17, characterized in that in order to enable the initial fresh gas to impinge more effectively aGainst the upper surface of the piston head portion, the piston head portion is bevelled off at its peripheral edges.
 22. A two-cycle internal combustion engine according to claim 17, characterized in that in order to enable the initial fresh gas to impinge more effectively against the upper surface of the piston head portion, the piston head portion is slightly curved outwardly.
 23. An internal combustion engine according to claim 20, characterized in that at the height of the inner top edge of the inlet port openings, the cylinder means is divided into two parts by an annular gap extending substantially perpendicular to the axis of the cylinder means.
 24. An internal combustion engine according to claim 23, characterized in that the annular gap is delimited on the lower cylinder part by at least one flat annular member which rests on lands between the inlet port openings.
 25. A two-cycle internal combustion engine according to claim 14, characterized in that guide and spacer rib means forming part of said gas guide means are arranged at the annular member.
 26. A two-cycle internal combustion engine according to claim 25, characterized in that several such annular members provided with guide rib means are arranged one above the other between the cylinder parts.
 27. An internal combustion engine according to claim 23, characterized in that one of the cylinder parts is a working cylinder part and in that the working cylinder part is provided at its end faces with a profile ending in an acute angle for the admission and exhaust.
 28. A two-cycle internal combustion engine according to claim 6, characterized in that the at least one exhaust port opening has an at least approximately triangular cross-section which is opened first by the piston at the larger width thereof.
 29. A two-cycle internal combustion engine according to claim 6, characterized in that the at least one exhaust port opening has an at least approximately trapezoidally shaped cross-section which is opened first by the piston at the larger width thereof.
 30. An internal combustion engine according to claim 1, characterized in that at the height of a controlling upper edge of the exhaust port means, the cylinder is divided axially by gaps perpendicular to the axis of the cylinder, the gap in the axial direction being smaller than the height of a piston ring.
 31. An internal combustion engine according to claim 30, characterized in that the last of the parts of a divided cylinder, which are provided with profiled inlet or exhaust port means, is made of a metal which has better thermal conductivity than the expansion part of a working cylinder means.
 32. A two-cycle internal combustion engine according to claim 31, characterized in that said last part is made of a metal which is lighter in weight and has a better thermal conductivity than cast-iron.
 33. A two-cycle internal combustion engine according to claim 1, characterized in that edges delimiting the inlet and exhaust port means are rounded off.
 34. An internal combustion engine according to claim 1, characterized in that particularly in the case of an axially divided cylinder, the part of a cylinder which is provided with the port means, and which is correspondingly profiled on its end face is manufactured by a shell molding process.
 35. A two-stroke internal combustion engine according to claim 17, characterized in that the exhaust port means are also so shaped that they open and close with their maximum width in the direction of the periphery of the cylinder means.
 36. A two-stroke internal combustion engine according to claim 13, characterized in that the inlet port openings are disposed in a ring around the cylinder liner in such a manner that the total width of their intermediate lands at the commencement of the opening amounts to only a small fraction of the periphery of the cylinder means.
 37. An internal combustion engine according to claim 1, characterized in that in order to enable the initial fResh gas to impinge more effectively against the upper surface of the piston head portion, the piston head portion is bevelled off at its peripheral edges.
 38. An internal combustion engine according to claim 1, characterized in that at the height of the inner top edge of the inlet port means, the cylinder is divided into two parts by an annular gap substantially perpendicular to the axis of the cylinder, one of said cylinder parts being a working cylinder part.
 39. An internal combustion engine according to claim 38, characterized in that the annular gap is delimited on the lower cylinder part by at least one flat annular member which rests on the lands between the inlet port openings.
 40. A two-cycle internal combustion engine according to claim 39, characterized in that guide and spacer rib means forming part of the gas guide means are arranged at the annular member.
 41. A two-cycle internal combustion engine according to claim 39, characterized in that several such annular members provided with guide rib means are arranged one above the other between the cylinder parts.
 42. An internal combustion engine according to claim 38, characterized in that the working cylinder part is provided at the end faces for the admission and exhaust, with a profile ending in an acute angle. 